FR2680909A1 - Rack for storage or for transport of nuclear fuel and its method of manufacture - Google Patents

Abstract

In order to produce a storage or transport rack for nuclear fuel, of compact type, tubular cylinders (10) of square cross-section are arranged in staggered manner by welding connection pieces (12) onto the accessible outer faces of the cylinders. During this assembly, groups of four plates (20) made of a neutron capture (poisoning) material are arranged in the cavities (16) formed between the cylinders (14) which plates separate these cavities (16) from each of the adjacent cavities formed inside the cylinders (10). The plates (20) are held in place by mutual interlocking, by virtue of complementary notches formed in their longitudinal edges.

Description

RACK FOR STORAGE OR TRANSPORT OF
NUCLEAR FUEL AND MANUFACTURING METHOD THEREOF
DESCRIPTION
The invention relates to a storage or transport rack designed to receive nuclear fuel assemblies after they have been irradiated in a nuclear reactor. The invention also relates to a method of manufacturing such a rack.

 Nuclear power plants generally include one or more swimming pools, of various shapes, in which are placed racks designed to receive spent nuclear fuel assemblies, after these have been extracted from the reactor core. Each rack delimits a certain number of vertical cells, of substantially square section.

 In practice, a rack generally consists of several juxtaposed modules comprising several tens of cells, the sizes of these modules being chosen according to the shape of the area available in the pool. By convention, the word "rack" designates throughout the text a module whose shape and dimensions can be any.

 In order to increase the compactness of the racks, the distances between the adjacent cells are reduced as much as possible. In order for this reduction in the distances between cells to be possible while preserving a sufficient margin with respect to the criticality, it is necessary to interpose between the cells of the walls made of a material with a high power of absorption vis-à-vis neutrons, usually called neutron absorbing material.

 In addition, the nuclear fuel storage racks must be designed to withstand the specific conditions imposed by the operation of the nuclear power plant, throughout its lifetime, that is to say forty 'years. Thus, the racks must have good resistance to corrosion in pure water or with added boric acid, as well as good dimensional and structural stability under radiation. In addition, their structure must ensure the integrity of the nuclear fuel stored therein under all circumstances, in particular in the event of an earthquake.

 However, neutron-absorbing materials are often difficult to use, in particular with regard to assembly operations by bending, welding, riveting, etc., because of their lack of ductility and of their particular metallic structure subject to alterations. when heating or melting. Consequently, these assembly operations can reveal cracks, migrations of borides at the grain boundaries and residual stresses detrimental to the mechanical strength and to the corrosion resistance.

 These drawbacks relate in particular to borated stainless steels, which normally have very good corrosion resistance and very high stability under radiation, but which very poorly support the abovementioned assembly operations. Thus, the bending of these steels, especially for small radii of curvature, reveals cracks which are primers of corrosion and fractures. Furthermore, the welding of these steels reveals microcracks, also subsequent sources of corrosion and fractures, during the solidification of the molten metal associated with the presence of borides. Finally, operations such as bending, welding and riveting leave residual stresses detrimental to the strength of borated stainless steels, because the latter are subject to intergranular corrosion when they are under stress. This drawback is all the more unacceptable as the relaxation of these stresses by heat treatment is costly and difficult to implement on complex assemblies such as storage racks.

 Furthermore, the creation of very compact nuclear fuel storage racks, thanks to the use of neutron-absorbing materials, is a source of difficulty during assembly.

Indeed, unless implementing complicated and expensive equipment and procedures, it is not possible to carry out welding or riveting operations within an already formed network of cells.

 For reasons related to criticality and mechanical strength, in particular in the event of an earthquake, another constraint imposed on storage racks concerns the uniformity of the network formed by the rack.

In particular, it is desirable that the pitch between cells as well as the dimensions of the cells are constant or very close.

 Finally, it is also desirable that the assembly of the racks be made from standardized elements in order to facilitate manufacture and reduce costs.

 Document EP-A-0 107 889 describes three different embodiments of a nuclear fuel storage rack of the compact type, having a uniform network and the manufacture of which employs standardized elements.

 In the first embodiment described in this document, the rack is formed by the juxtaposition of tubular cylinders of substantially square section, one in two of which is coated over its entire periphery with a wall of neutron absorbing material. This wall therefore has, on each of the edges of the square section of the cylinder, folded parts with a small radius of curvature causing a risk of cracking and intergranular corrosion.

 In the second embodiment described in document EP-A-0 107 889, the rack is also formed by the assembly of tubular cylinders of substantially square section juxtaposed. In this case, each of the cylinders carries on two of its opposite external faces a wall of neutron absorbing material, which is held against the cylinder by metal strips welded to the latter. The relative arrangement of the adjacent cylinders is produced so that each of the cells defined inside the cylinders is surrounded on all its faces by a wall of neutron absorbing material. By eliminating any folding of the walls of neutron absorbing material, this embodiment avoids the drawbacks of the previous one.

 Finally, the third embodiment described in document EP-A-0 107 889 relates to a storage rack formed by the juxtaposition of tubular cylinders of substantially square section each carrying a wall of neutron absorbing material on one of its external faces. As in the previous embodiment, the assembly of the cylinders is carried out so that each cell is completely surrounded by walls of neutron absorbing material. This embodiment also avoids the drawbacks associated with folding the neutron absorbing material.

 However, all of the embodiments described in this document have in common that no fixing means are provided making it possible to connect the adjacent cylinders together. The integrity of the rack is therefore ensured only by a monobloc external frame surrounding all of the cylinders and by spacers placed between the adjacent rows of cylinders.

This gives the whole a relatively heavy structure and tends to increase the total thickness of the walls separating two adjacent cells. The compactness of the rack is therefore reduced. In addition, the failure to maintain the cylinders parallel to their longitudinal axis can result in one of them being driven by an assembly of nuclear fuel deformed under the effect of irradiation, when this assembly is extracted. rack. This can cause this cylinder or the adjacent cylinders to fall, which is obviously unacceptable.

The main object of the present invention is a rack for the storage or transport of nuclear fuel of a new type, having an even greater compactness and a lighter structure than that described in the document.
EP-AO 107 889, in which the integrity of the rack is preserved in all circumstances, this rack also having a uniform network, formed of standardized elements whose assembly can be carried out easily, and in which the walls of neutron-absorbing material do not undergo any operation of the folding, welding or riveting type which could weaken the mechanical strength.

 According to the invention, this result is obtained by means of a nuclear fuel storage or transport rack, comprising tubular cylinders of substantially square section, arranged to delimit juxtaposed cells, separated from one another by walls in neutron absorbing material, characterized in that the cylinders of substantially square section are assembled in staggered rows by connecting pieces, so as to define first inner cells to the cylinders and second outer cells to the cylinders, said walls of neutron absorbing material internally delimiting the second cells.

 In a preferred embodiment of the invention, each of the second cells is delimited by four substantially flat plates of neutron absorbing material, not linked to the cylinders and having adjacent edges fitted one inside the other and, preferably, on the connecting pieces.

 Depending on the case, the plates of neutron absorbing material are then either in contact with the external surfaces of the cylinders, or in contact with the connecting parts and spaced from these surfaces. In the first case, the plates have on their adjacent edges first notches ensuring mutual interlocking of the plates and second notches ensuring interlocking of the plates on the connecting pieces. In the second case, the plates have on their adjacent edges notches ensuring simultaneous interlocking of the plates and interlocking of the plates on the connecting pieces.

 Preferably, the connecting pieces consist of plates which are welded to the external surfaces of the cylinders and distributed along these cylinders, so as to form crosses connecting their adjacent cylinders, of substantially square section.

 So that the internal dimensions of the two series of cells are substantially the same, different arrangements can be provided depending on whether the plates of neutron absorbing material are or are not directly in contact with the external surfaces of the cylinders.

 Thus, when the plates of neutrophage material are in contact with the external surfaces of the cylinders, flat connection plates are used and the plates of neutron absorbing material have a thickness substantially equal to the sum of the thicknesses of the cylinders and of the plates.

 When the plates of neutron absorbing material are spaced from the outer surfaces of the cylinders, a first solution consists in using flat plates and plates of neutron absorbing material having substantially the same thickness as the cylinders. A second solution also consists in using plates having the shape of a flattened S, so that the end regions of two opposite faces of each plate, facing the second cells, are located substantially in the same plane. The plates of neutron absorbing material then have a thickness substantially equal to the sum of the thicknesses of the cylinders and the plates.

 The neutron absorbing material can in particular be a steel alloyed with boron, while the cylinders are made of stainless steel.

 The invention also relates to a method of manufacturing a rack for storing or transporting nuclear fuel. This process, consisting in assembling tubular cylinders of substantially square section, to form juxtaposed cells, separated from each other by walls of neutron absorbing material, is characterized in that connecting pieces are attached to each cylinder, before to assemble the cylinders in staggered rows, by means of these connecting parts, so as to define first internal cells to the cylinders and second external cells to the cylinders, said walls of neutron absorbing material being placed between the cylinders, so as to delimit internally the second cells.

We will now describe, by way of nonlimiting example, different embodiments of the invention, with reference to the accompanying drawings, in which
- Figure 1 is a top view, in which the right and left halves represent sections taken at two different levels, illustrating the assembly of four cylinders with a substantially square section according to a first embodiment of the invention;
- Figure 2 is a top view showing a part of a storage rack produced according to a second embodiment of the invention;
- Figure 3 is a perspective view exploded different parts of the storage rack illustrated in Figure 2, before assembly; and
- Figure 4 is a top view comparable to Figure 2 illustrating a third embodiment of the invention.

 As illustrated in FIG. 1, a nuclear fuel storage rack according to the invention comprises tubular cylinders 10, of substantially square section, which are assembled in staggered rows by means of connection plates 12, so as to define two networks superimposed on cells forming a checkerboard view from above. A first of these networks comprises first cells 14 inside the cylinders 10, while the second network is formed by second cells 16 outside these cylinders. The superposition of the two networks of cells 14 and 16 forms a square-pitch network in which each of the cells has substantially identical internal dimensions, making it possible to introduce an assembly of nuclear fuel therein.

 The tubular cylinders 10, as well as the connecting plates 12 are made of a non-neutron absorbing material such as stainless steel. In addition, the cylinders 10 on the one hand, and the connecting plates 12, on the other hand, are all identical.

 In the embodiment illustrated in Figure 1, the connecting plates 12 are flat and fixed by welds 18 on the outer surfaces of the cylinders 10, near the corners of these cylinders. More specifically, two adjacent cylinders 10 are connected by their corners by means of two alternating series of plates 12, arranged in a cross view from above, the plates of a first series connecting two of the faces of the cylinders, adjacent to the corners concerned, then that the pads of the other series connect the two other faces of the cylinders, adjacent to these corners. The plates 12 serving to connect by their corners two adjacent cylinders 10 are distributed uniformly, preferably, over the entire length of these cylinders.

 As will be seen in more detail below, the connection plates 12 of each series are welded at 18 to one of the cylinders 10, before the latter are assembled together.

 As also illustrated in FIG. 1, the rack according to the invention comprises, in addition to the tubular cylinders 10 and connecting plates 12, plates 20 of neutron absorbing material, which are arranged outside the cylinders 10, so to delimit internally the second cells 16. More precisely, each of the second cells 16, only one of which is shown in FIG. 1, is delimited by four plates 20, each of these plates 20 being held against the external surface of the one of the cylinders 10 adjacent to this cell 16. The plates 20 thus form walls of neutron absorbing material which separate each of the cells 16 from the cells 14 which are adjacent to it.

 The mounting of the plates 20 of neutron absorbing material and the holding in place of these plates are ensured without any connection or fixing of these plates on the cylinders 10. For this, the adjacent longitudinal edges of the plates 20 situated in the corners of the cells 16 cooperate with each other by complementary notches, similar to the notches 24 illustrated in FIG. 3. These complementary notches allow mutual interlocking of the four plates 20 delimiting each of the second cells 16, so that these plates are immobilized one relative to the others in the longitudinal direction of the cell, and that the plates 20 remain applied against the outer faces of the tubular cylinders 10.

 In the embodiment of FIG. 1, in which the plates 20 are directly in contact with the external surfaces of the adjacent cylinders 10, these complementary notches formed in the adjacent longitudinal edges of the plates 20 are completed by notches 22, also formed in these longitudinal edges of the plates 20, in line with the plates 12 ensuring the connection between the adjacent cylinders 10. These notches 22 are illustrated on the right half of FIG. 1. They make it possible to immobilize the plates 20 in translation parallel to the axis of the cells 16 relative to the cylinders 10.

 The dimensions of the nuclear fuel assemblies received in the cells 14 and 16 being the same, it is desirable that the dimensions of these cells are also substantially the same. In the embodiment of FIG. 1, in which the cylinders 10 are connected by flat plates 12 and in which the plates 20 are in direct contact with the external surfaces of the cylinders, this result is obtained by giving each of the plates 20 a thickness substantially equal to the sum of the thicknesses of the connecting plate 12 and the wall of the cylinder 10.

 A second embodiment of the invention will now be described, with reference to FIGS. 2 and 3, in which the plates 20 of neutron absorbing material are no longer directly supported on the external faces of the tubular cylinders 10, but on the plates of connection 12. This characteristic makes it possible to eliminate the notches 22 in FIG. 1 and to make the cells 16 perfectly smooth.

 Furthermore, the planar shape of the connection plates 12, as well as the arrangement of these plates are completely identical to those which have been described previously with reference to FIG. 1.

 To take account of the spacing existing in this case between the plates 20 and the adjacent cylinders 14, obtaining substantially equal dimensions for the cells 14 and 16 is ensured by giving each of the plates 20 a thickness substantially equal to L ' wall thickness of the tubular cylinder 10.

 In this second embodiment, since the holding of the plates 20 relative to the tubular cylinders 10 is no longer ensured by the notches 22 of FIG. 1, the cutting of the adjacent longitudinal edges of the plates 20 is carried out, as in more precisely illustrates FIG. 3, so that each of the notches 24 formed in any one of these longitudinal edges receives both a tongue 26 formed between two notches 24 on the adjacent longitudinal edge of the adjacent plate 20, as well as one of the connecting plates 12.

 Although this arrangement is not compulsory, it provides better security against any risk of incidental or accidental dislocation of the plates 20 during handling.

 The exploded perspective view of Figure 3 allows to understand the manufacturing process of the rack according to the invention.

 Firstly, to each of the tubular cylinders 10 are welded four series of plates 12, each series of plates corresponding to one of the corners of the cylinders of substantially square section.

 More specifically, the plates 12 are welded to the standard tubular cylinders 10 as illustrated for the cylinders 10-1, 10-2, 10-3 and 10-4 in FIG. 3. In this case, two series of plates 12 -1 and 12-2 are welded on the same face of each cylinder of substantially square section, so as to protrude beyond the two corners adjacent to this face. The other two series of plates 12-3 and 12-4 are welded on the two faces adjacent to the face carrying the plates 12-1 and 12-2, so as to protrude beyond the two other corners of the cylinder. The plates 12-1 and 12-2 are therefore located in the same plane, while the plates 12-4 are parallel to the plates 12-3 and perpendicular to the above-mentioned plane.

 As will be better understood by observing FIG. 2, the tubular cylinders intended to be placed on the different sides and in the corners of the rack are distinguished from the standard cylinders by implantations of slightly different plates 12.

There are thus four other types of layout for the cylinders which are on the four sides of the rack and two other types of layout for the four cylinders which are at the corners of the rack.

 In addition, the peripheral closure of the rack requires the use of flat side plates 28 and corner plates 30 in the form of a V, which appear in FIG. 2.

 When the different tubular cylinders 10 have been equipped with their four series of plates 12, as described above with reference to FIG. 3, the tubular cylinders are assembled in groups of three, the orientations of the different series of plates being the same for each of the three cylinders concerned.

 More precisely, in the embodiment illustrated in FIG. 3, the tubular cylinders 10-2 and 10-3 are fixed to each other by respectively welding the plates 12-2 of the cylinder 10-2 on the face of the cylinder 10-3 facing forwards in FIG. 3 and by welding the plates 12-4 of the cylinder 10-3 on the face facing to the right of the cylinder 102. In addition, the tubular cylinder 10-4 is fixed to the tubular cylinder 10-3 by welding the plates 12-1 of the cylinder 10-4 on the side facing the front of the cylinder 10-3 and by welding the plates 12-3 of the cylinder 10-3 on the side facing the the left side of cylinder 10-4. It should be noted that the various welds can be made without difficulty since they relate to the external faces of the tubular cylinders, accessible without it being necessary to enter a cell of the rack.

 When the three tubular cylinders 10-2, 10-3 and 10-4 have been assembled, the four plates 20 made of neutron absorbing material intended to delimit this alveolus are introduced into the cell 16 then partially formed between these three cylinders. The notches 24 and the tongues 26 formed on the adjacent longitudinal edges of the plates 20 then ensure the immobilization of these plates relative to one another and relative to the plates 12-3 and 12-4 of the tubular cylinder 10-3.

 The cell 16 thus produced is then closed by fixing the fourth tubular cylinder 10-1 on the cylinders 10-2 and 10-4 which are adjacent to it.

More specifically, the plates 12-1 of the cylinder 10-1 are welded on the face facing the front of the cylinder 10-2 and the plates 12-3 of the cylinder 10-2 are welded on the face facing the left of the cylinder 10-1, in order to secure the latter to the cylinder 10-2.

Furthermore, the plates 12-2 of the cylinder 10-1 are welded on the face facing the front of the cylinder 10-4 and the plates 12-4 of the cylinder 10-4 are welded on the face facing the right of the cylinder 10-1, so as to secure the latter to the cylinder 104.

 It is important to observe that the orientations of the various plates 12 welded to the cylinders 10 are such that the welds which must be carried out in order to secure the cylinder 10-1 to the cylinders 10-2 and 10-4 are all made on the faces of these cylinders which are turned outwards and therefore perfectly accessible at this stage of assembly.

 The operation which has just been described for the assembly of four adjacent tubular cylinders 10, allowing the formation of a cell 16 delimited by four plates 20 of neutron absorbing material, is repeated step by step, until obtaining a storage or transport rack with the required dimensions. More specifically, the manufacture of the rack begins with the complete production of one side thereof, then the various rows of cells constituting the rack are carried out gradually to the other side of the rack.

 In Figure 4, there is shown a third embodiment of the invention, which borrows from the first embodiment described with reference to Figure 1 the thicker nature of the plates 20 of neutron absorbing material and the second embodiment described with reference to FIGS. 2 and 3, the completely smooth character of the cells 16.

 In order to obtain these two characteristics simultaneously, while keeping substantially identical dimensions for the cells 14 and 16, connection plates 12 are used in this case which, instead of being flat, have the shape of a flattened S . More specifically, each of the plates 12 has two end parts which are substantially flat and parallel to each other and a central curved connecting part, so that the faces of the two end parts which are substantially planar and which face towards the cells 16 are located substantially in the same plane. Thanks to this characteristic, dimensions of the cells 16 substantially identical to those of the cells 14 can be obtained by giving each of the plates 20 of neutron absorbing material a thickness substantially equal to the sum of the thicknesses of the wall of the tubular cylinder 10 and of the wafer. 12.

 As illustrated in FIG. 4, the closure plates 28 and 30 of the rack are also modified so as to have substantially flat end portions, offset towards the inside of the rack by a value corresponding to the thickness of these plates.

 As in the previous embodiments, a particularly compact rack is thus obtained, having a uniform network formed of standardized elements, and in which the plates of neutron absorbing material do not undergo any treatment such as bending, welding or riveting, liable to weaken them. . In addition, the assembly procedure described with reference to Figure 3, can be used in all cases and ensures the integrity of the rack, without the need for tools or equipment. complex procedures.

 Of course, the invention is not limited to the embodiments which have just been described by way of examples, but covers all the variants thereof.

Thus, if the invention is particularly suitable for the production of a nuclear fuel storage rack, it can also be used for the production of a rack for transporting this fuel.

Claims (15)

 1. Nuclear fuel storage or transport rack, comprising tubular cylinders (10) of substantially square section arranged to delimit juxtaposed cells, separated from each other by walls (20) of neutron-absorbing material, characterized in that the cylinders (10) of substantially square section are staggered by connecting pieces (12), so as to define first cells (14) inside the cylinders and second cells (16) outside the cylinders, said walls (20 ) in neutron absorbing material internally delimiting the second cells.  2. Rack according to claim 1, characterized in that each of the second cells (16) is delimited by four substantially flat plates (20) of neutron absorbing material, not linked to the cylinders (10) and having adjacent edges nested one in others.  3. Rack according to claim 2, characterized in that the adjacent edges of said plates (20) are also fitted onto the connecting pieces (12).  4. Rack according to claim 3, characterized in that said plates (20) of neutron absorbing material are in contact with external surfaces of the cylinders (10), and have on their adjacent edges first notches (24) ensuring a nesting- mutual muting of the plates and of the second notches (22) ensuring an interlocking of the plates on the connecting pieces (12).  5. Rack according to claim 3, characterized in that said plates (20) of neutron absorbing material are in contact with the connecting parts (12) and spaced from the outer surfaces of the cylinders (10), these plates having on their adjacent edges notches (24) simultaneously ensuring mutual interlocking of the plates and an interlocking of the latter on the connecting pieces.  6. Rack according to any one of the preceding claims, characterized in that the connecting pieces are plates (12) welded to the outer surfaces of the cylinders (10) and distributed along the latter, so as to form crosses connecting by their corners the adjacent cylinders, of substantially square section.  7. Rack according to claim 6, combined with claim 4, characterized in that said plates (12) are flat and in that the plates (20) of neutron absorbing material have a thickness substantially equal to the sum of the thicknesses walls of cylinders (10) and pads.  8. Rack according to claim 6, combined with claim 5, characterized in that said plates (12) are flat and in that the plates (20) of neutron absorbing material and the walls of the cylinders (10) have substantially the same thickness.  9. Rack according to claim 6, combined with claim 5, characterized in that said plates (12) have the shape of a flattened S, so that end regions of two opposite faces of each plate, turned towards the last cells (16), are located substantially in the same plane, and by the fact that the plates (20) of neutron absorbing material have a thickness substantially equal to the sum of the thicknesses of the walls of the cylinders (10) and of the plates (12).  10. Rack according to any one of the preceding claims, characterized in that the neutron absorbing material is a steel alloyed with boron.  11. Rack according to any one of the preceding claims, characterized in that the cylinders (10) are made of stainless steel.  12. A method of manufacturing a rack for storing or transporting nuclear fuel, consisting in assembling tubular cylinders (10) of substantially square section, to form juxtaposed cells, separated from each other by walls (20) in neut rophage material, characterized in that one fixes on each cylinder (10) connecting pieces (12), before assembling the cylinders in staggered rows, by means of these connecting pieces, so as to define first cells (14) inside the cylinders and second cells (16) outside the cylinders, said walls (20) of neutron absorbing material being placed between the cylinders, so as to delimit the second cells (16) internally.  13. Method according to claim 12, characterized in that the walls (20) of neutron absorbing material delimiting one of the second cells (16) are placed after the assembly of three of the cylinders (10) adjacent to this second cell.  14. Method according to claim 13, characterized in that the walls of neutron absorbing material delimiting each of the second cells are formed by four plates (20) held by mutual interlocking and by interlocking on the connecting pieces (12), without connection with the cylinders (10).  15. Method according to any one of claims 13 and 14, characterized in that each of the second cells (16) is closed by means of a fourth cylinder (10-1), which is assembled with the two cylinders adjacent (10-2, 10-4) delimiting this second cell, by fixing the connecting pieces (12) on accessible external surfaces of this fourth cylinder and these two adjacent cylinders.